In general, image-forming processes in a copying machine and a printer that use an electrophotographic process are carried out in the following manner: That is, by exposing a charged photosensitive drum, an electrostatic latent image is formed, and by allowing toner to adhere to this electrostatic latent image, a toner image is formed, and an image is formed by transferring this toner image onto a sheet of recording paper. In this electrophotographic process, with respect to the method for forming a toner image, as shown in FIG. 8, a system using a developing roller 1 (contact developing system) is adopted. Toner 3, charged and supplied onto the developing roller 1, is transported in a right rotating direction following the rotation of the developing roller 1, and is allowed to pass between the developing roller 1 and a toner-layer thickness regulating member 2 so as to be adjusted to a predetermined layer thickness. The toner 3 is transported to a developing area at which the developing roller 1 and a photosensitive member 4 are made face to face with each other, following the rotation of the developing roller 1; thus, the toner is allowed to adhere to the electrostatic latent image on the photosensitive member 4 by a function of a bias potential applied between the developing roller 1 and the photosensitive member 4 so that a toner image is formed.
As shown in FIG. 9, this developing roller 1 has a multi-layer structure in which, on a core metal member 11 serving as a supporting shaft, a base rubber layer 12, an intermediate layer 13 and a surface layer 14 are formed in this order, and carbon black is dispersed in each of the base rubber layer 12, the intermediate layer 13 and the surface layer 14, so as to adjust each of the layers to an appropriate conductivity.